1. Field of the Invention
The present invention relates to an asynchronous transmission device and asynchronous transmission method which transmit information between circuits having different clocks.
2. Description of Related Art
In an asynchronous transmission device in which a transmission side circuit transmits a notification signal of an event, etc. to a reception side circuit, in case the transmission side circuit and the reception side circuit operate with clock frequencies different from each other, so as to prevent the reception side circuit from coming into the metastable state, it is necessary to synchronize the notification signal in the reception side circuit.
Firstly, the first conventional asynchronous transmission device will be explained. In the first conventional asynchronous transmission device, a synchronization circuit is used in a reception side circuit for an input notification signal. The synchronization circuit has synchronization FFs (Flip-Flops) of suitable number of stages which are serially connected, and performs re-timing using a reception clock to synchronize the input notification signal.
When the number of stages of synchronization FFs is increased, the probability of synchronization failure can be decreased, while the synchronization processing overhead is increased. The synchronization processing overhead is a latency between a notification signal transmitted from a transmission side circuit and a notification signal retained to a reception side circuit, and is a latency corresponding to the number of stages of synchronization FFs. Accordingly, the number of stages of synchronization FFs, which is the number of stages of FFs necessary for synchronization, is set to be the minimum value satisfying the allowable probability of synchronization failure. For example, in case the clock frequency is several hundred MHz, the number of stages of synchronization FFs is set to be approximately three to five.
In case there exist a plurality of notification signals from a transmission side circuit to a reception side circuit, the reception side circuit has above-described synchronization circuits for the respective input notification signals.
As a conventional technique related to the present invention, there is known a patent document 1 (Jpn. Pat. Appln. Laid-Open Publication No. 01-128153). In a control signal transmission system between asynchronous devices, a transmission side device transmits a trigger signal indicating that writing a control signal to be transmitted to a transmission register is ended, while a reception side device performs gating for the input control signal using the trigger signal which is synchronized by a synchronization circuit.
However, when employing above-described synchronization circuit, the synchronization processing overhead comes to be at least a latency corresponding to the number of stages of synchronization FFs.